Fusion of part of the spine for instability, infection, tumor, degeneration and deformity has become a recognized surgical procedure for spine surgeons. The three approaches to the spine to perform these procedures are anterior, posterior and lateral, with the posterior being most common. It has become increasingly recognized that fusion between two adjacent vertebral bodies in the space occupied by the disc is desirable for biomechanical, neurophysiological and anatomical reasons. This “interbody fusion” is biomechanically advantageous because the area to be fused is subjected to compressive loads rather than tensile forces as in the case for posterior element fusions. It also offers the best way to restore or maintain the opening of the neuroforamina and to restore or maintain lumbar lordosis. Quite often spinal deformity correction cannot adequately be performed without interbody surgery.
Lumbar interbody fusion is usually performed from either the anterior or posterior approaches although lateral approaches are occasionally used as well. The goals of interbody fusion are as follows: I. To maintain sagittal and frontal plane alignment of the spine, 2. To maintain or restore intervertebral space dimension, 3. To achieve a solid fusion. To this end a number of surgical techniques and graft materials have been utilized to attain a safe and successful pain relieving fusion.
Reduced to the most rudimentary level, anterior interbody fusion is performed by removing all or part of the intervertebral disc, preparing the bony interspace and placing graft material into the space. Supplemental fixation devices are often used to keep the graft from “backing out,” getting crushed by the compressive and complex loads and to help maintain alignment while the fusion takes place. In prior years various bone graft materials utilized have been bovine zenograft; allograft tibia, fibula, femur, iliac crest and autograft iliac crest, and fibula. Success rates in terms of achieving fusion varied from 63-95%. They all shared the problems of graft failure from dislodgement, fragmentation, failure to achieve fusion or loss of alignment from subsidence.
More recently threaded cylindrical “cages” made from either titanium or fresh frozen allograft femoral diaphysis have been used. The stability provided by the threaded design allowed these implants to be used as a “stand alone” device not requiring further accessory stabilization. However, there have been increasing reports of non-union when initially fusion was thought to have occurred and subsidence with sinking of the implants into the vertebral body.
Threaded cylindrical cages require tapping to insert the cage. Tapping causes destruction of the supportive end plates of the vertebra allowing subsidence or “sinking in” of the implant into the body of the vertebra. This causes a loss of height of the spinal column with narrowing of the foramina and potentially compression of the exiting nerve root. There is also a flattening of the lumbar lordosis resulting in lower back pain. Furthermore the long term effects on the body from the entrapped metal implant and its local effects on the spine are unknown. If removal becomes necessary due to pain or infection the metallic cages are virtually impossible to remove without endangering the greater vessels overlying the anterior spine and necessitating massive destruction of the involved vertebrae. This creates an almost impossible situation to re-stabilize the spine. Consequently there is increasing awareness that a better design and conceived bone or non metallic biomaterials implant interbody fusion technique is necessary.
In this surgical procedure as it generally and currently is practiced, the body is entered and the spine is accessed from its anterior or lateral side. Layers of tissue surrounding the spine have to be opened carefully so that adjacent nerves and blood vessel structures, including the aorta and inferior vena cava are not ruptured and preferably are not immobilized more than necessary. Once the spine is accessed, the intervertebral disc between two adjacent vertebra must be carefully and safely removed.
A common implant used in intervertebral fusion is a femoral diaphyseal ring both freeze dried and fresh frozen allograft. This is used both for structural support to maintain the intervertebral space and to promote bone growth to fuse two adjacent vertebra together. A serious problem exists with respect to the surgical procedure of accessing, preparing the space between adjacent vertebrae, and in inserting and positioning the femoral ring allograft. One company's answer is a triple jointed spreading device. A pair of spreading levers, referred to as distractor blades, are laterally offset mounted from the centerline of the last joint of the triple jointed spreading device. The spreading levers are thin, and relatively narrow and thus potentially and actually unstable increasing the danger of inadvertent injury of surrounding tissues. The spreading device is bulky, long and since it extends straight into the space between the two vertebra, it blocks the approach and takes up valuable space and blocks needed vision into the critical operative area. The offset is for the purpose of inserting a second set of pliers-action implant holder to just clear the offset.
Unfortunately, the pliers-action implant holder must push the implant into a space which has a height taken up by the thickness of the distractor blades. This poses the danger again of dislodging the spreader device potentially causing tissue and vascular injury. Aside from the inherent instability of having spreaders, the use of the triple jointed spreading device requires excessive spreading in order to achieve its goal of providing working room to shape the interspace. Overspreading of the interspace can damage, even fracture, the vertebra. It also potentially damages the discs above and below the working level and can cause neurological injury with foraminal compression.
In some cases, a spacing tool is used or inserted while the intervertebral space is distracted with the three joint distractor. The spacing tool conventionally used is a rectangular paddle mounted at the end of a straight handle. The spacing tool is cumbersome because the handle which extends straight from the operational area further gets in the way of the surgeon. Insertion of the spacing tool is also cumbersome as it can be inserted only if the size of the inter vertebral opening exceeds the clearance size of the width of the rectangular paddle. Because the natural disc space is biconcave, the surgeon is faced with the problem of fitting a rectangular profile object into an elliptical space. This results in poor contact between the end plates of the adjacent vertebra and the surface of the bone graft which militates against successful fusion.
If the surgeon chooses to carve a rectangular space to accommodate the spacer or the graft, he must necessarily remove a great deal of the all important end plate thereby weakening the most structurally supportive part of the vertebra. This then subject the fusion site to subside and thereby resulting in unwelcome deformity with loss of normal spinal curvature and foraminal narrowing. The operation should involve only enough access to accomplish the objective of safely preparing the interspace and inserting a graft. Aspects of attaining this objective includes elimination of excessive spreading of adjacent vertebrae, enabling the surgeon to operate with as full an amount of control over the surgical field as is possible, as full an amount of vision into the surgical field as is possible, reduction of obstructions into the surgical field, and importantly, supplying the surgeon with tools which enable complete force control and selection. Proper surgical tools should lend themselves for automatic adaptation for patients of different size and of different complications. The excess force and over spacing should be eliminated.
The shape, stability, handling and force used in preparation and insertion of the implant is also a problem with spine fusion surgery. Where the implant is to be inserted, and particularly where the adjacent vertebrae are under compressive force, it may be expelled from the intervertebral space as the result of such compressive forces. The insertion using a poor grasping tool typically allows rotation or lateral displacement of the graft before the surgeon has a chance to make final placement and secure it.
The degree of spreading of two adjacent vertebrae away from the intervertebral space should be limited to avoid trauma to surrounding areas, yet enable the surgeon to access the area for removal of the disc and shaping of the interspace. Current surgical instruments available for this purpose do not enable both access, full disc removability and interspace shaping without obstructing the surgical field or unduly lengthening the time required for the procedure.
Currently used instruments to prepare the interspace such as osteotomes, chisels, curettes, rongeurs and high speed drills all have some application as well as drawbacks. The use of any combination of these instruments still does not achieve the goal of a safe, quick and anatomically shaped interspace to match a like contoured implant. Extensive use of curettes is time consuming and leaves an uneven end plate surface. Osteotomes and chisels are often too short for safe application and will not result in the ability to perform precision work. High speed drills can be quick, but can easily wrap up adjacent soft tissue resulting in catastrophic vascular injury. It is also difficult to control in the more posterior recesses of the interspace and can transgress the posterior rim and inadvertently enter the spinal canal and cause permanent neurological injury.
Another feature lacking in surgical instruments is the ability to remove instruments in a way which will not encourage side to side loosening. When an inserted instrument becomes jammed, lateral movement or force will tend to damage the surrounding areas. The surgeon's lack of control over exit angle as well as entry angle is a problem in performing this type of procedure. This is especially complicated by the fact that major blood vessels lie to either side of the operative area.
The obstruction of the surgical field is another problem. Extremely long, complicated instruments, especially those instruments which have hand engagement members located far from the surgical field, cause a significant obstruction problem. This is compounded by instrumentation which is used to hold the adjacent vertebrae apart. At the point in the procedure where the implant is to be implanted or implaced, a large number of instruments, particularly long, obstructing instruments, may be simultaneously present. The resulting obstruction is both significant and hazardous.
Implants, such as femoral diaphyseal rings currently used give the surgeon problems of (1) rotating in the interspace during insertion, (2) not remaining positioned properly to the surgical instrument utilized for implantation and fixation, (3) backing out of the interspace, (4) fracturing during insertion and (5) failure to achieve fusion.
Ideally, the implant would be contoured to restore the lumbar lordosis and match the shape of the anatomical interspace. It should have desired surface etching to securely mate to an entry instrument and resist extrusion and rotational shear forces. It should also have a surface design to increase the surface area in order to promote more rapid bone growth. No design has yet provided a solution to these problems in the allograft diaphyseal ring implant field.
What is therefore needed is a set of surgical instruments which can be utilized for spine fusion operations which reduce the visual and manual obstruction in the surgical field, give the surgeon more options for manipulation, better secure the implant on insertion and fixation, better orient the implant, enable a lesser magnitude of force to be applied to the procedure, and which enables the procedure to proceed in less time, more safely, and with better surgical control.
Other needs include anything which will reduce time during the operation, especially time spent in (1) removal of cartilage material, (2) shaping the intervertebral disc space to accept the implant, (3) selecting the correct sized implant for insertion to thus eliminate as far as possible having to remove the implant and increasing the possibility that it may be damaged from removal, or repeated re-insertions, and (4) further shaping the implant while the patient is in the midst of the operation extends the danger to the patient, the cost of operating theater time, creating the probability even in the hands of skilled surgeon that the implant may be over adjusted or improperly adjusted followed by improper implantation because no other implant was available or by wasting of a valuable implant. Not included in the list above are probabilities of having to re-set up for cartilage removal, as well as having to set up again for re-doing any porion of the operative procedure. A needed system should insure a proper fit, eliminate wasted time, and place the surgeon in a position to exert better management over the operative procedure.